Jesus Christ, I was just scrolling through my writing notes and ideas, and suddenly got struck so hard by this note I wrote:
post-S15E18 and pre-S15E20, but Jack-focused Jack is more anxious and guilty than Dean, thinking/knowing Cas is dead because of him (and Dean thinks it's because of him). Without telling Sam and Dean, Jack tries to go to Empty to save Cas but fails, he's wounded badly and still awfully guilty and in pain. Dean has to help him understand it's not his fault (and in the process Jack would make him understand it's not his either, Cas did what he wanted to do). The two of them have to make an effort to support each other during this extremely hard time after Cas's gone, building a really strong bond. And when Dean is very outright willing to self-sacrifice for an extremely dangerous plan to save Cas, Jack says to him: "I already lost one father, don't make me lose another. Please." So he stops. At least gives the whole plan a second thought. He promises Jack he will try his best to find a different way first, he wouldn't sacrifice himself unless they're absolutely sure that's the very last thing they can try. (And Dean asks Jack to promise the same.)